Immunoassays for benzoylecgonine are described in U.S. Pat. No. 3,888,866, issued June 10, 1975. The immunoassays disclosed in the patent involve the use of either a stable free radical detector (the FRAT technique) or the use of enzymes as detector moieties (EMIT). A further disclosure to such immunoassays for benzoylecgonine is found in U.S. Pat. No. 3,690,834, issued Sept. 12, 1972.